CAIRO, Egypt -- Suspected Muslim radicals answered the Egyptian government's crackdown on terrorism with a bold assassination attempt yesterday that killed four people and wounded 15, including their target, a top minister.

A bomb strapped to a parked motorcycle exploded near the city's main Tahrir Square just as a motorcade passed carrying Hassan el-Alfy, the minister of interior. He is in charge of combating the 19-month wave of attacks by Islamic fundamentalists.

Mr. el-Alfy was wounded in the arm and later appeared on national television from his hospital bed to denounce the attackers as "killers and butchers who have no religion."

His personal bodyguard and three pedestrians in the area were killed by the blast, which was powerful enough to topple a nearby tree.

"I saw Monsour lying on the ground. He got something right on his heart. His brother, Ali, was cut in three pieces," said Yassine el-Oraby, 22, a student at the American University in Cairo. The bomb exploded on a street beside the campus, where Monsour Abdel-Fatah, 45, and his brother made their living parking

students' cars for tips.

The third pedestrian was a Palestinian walking toward the university gate to visit a friend, according to Mr. el-Oraby, who said he dragged the wounded man into the safety of the campus and then watched him die.

The explosion was notable for its daring. Just as the government thought it was making progress in a tough campaign of arrests and executions, the blast targeted one of government's highest-ranking officials, on one of the city's busiest spots, in the height of the day.

"They are trying to send a message," said one bystander. "They're not beaten yet."

No group took responsibility for the explosion, but the government placed the blame on the Islamic Group, a radical Muslim organization bent on overthrowing the government.

In the past two years, the Islamic Group has been responsible for a string of attacks on Christians, tourists, police officers and government officials. The government has responded with the arrests of thousands of people and the execution of 15 convicted in speedy military trials.

"The people should retaliate," Mr. el-Alfy said on television last night. His head was bandaged, and his wounded arm was in a cast. "We urge all citizens to fight them."

The terrorist campaign, which began with a measure of public support against an unpopular government, has turned most Egyptians against the radicals, according to most indications.

"The people doing these things are not Egyptians. People hate it in Egypt," railed Said Ibrahim, 18, another student who heard the blast and saw people running in terror. "These things are sent to us from abroad."

Mr. el-Alfy's motorcade, containing his black Peugeot, a second car of bodyguards and motorcycle escorts, was attacked as he neared the Ministry of Interior.

Several witnesses said they heard shooting. A man who gave his name as Hassan said he saw attackers in jeans who were shooting machine guns, and he said that "the person who shot the minister then shot himself."

But the government denied that the attack included gunfire. Those shots that were heard were fired by police officers and bodyguards, according to the government statement.

According to government television, the bomb, containing TNT and ball bearings, was probably detonated by remote control.

Hours after the blast, police with riot gear had sealed off the street, and an armored personnel carrier was positioned nearby. The government of President Hosni Mubarak, who had watched his predecessor, Anwar el Sadat, gunned down by Muslim extremists in 1981, remains jittery about its inability to stop the wave of terrorism.

On Monday, shots were fired at a Nile River cruise boat. Although no one was hurt, such attacks have deeply cut into tourism, Egypt's chief source of revenue, and have led to open speculation about the future of Mr. Mubarak's regime.

The targeting of Mr. el-Alfy was both obvious and ironic. He was appointed minister of interior in April after serving as governor of an area of southern Egypt rife with fundamentalism. His ministry includes the police and security forces charged with combating the militants.

But since being appointed, Mr. el-Alfy had reduced the wide arrests that put thousands in jail, and he pledged to try to stop some of the human rights abuses that had accompanied the crackdown.

"Arrests do not take place without concrete evidence, witnesses and facts," Mr. el-Alfy had said. "Human rights are respected at all stages."

Extremist Muslim groups have been blamed for five other explosions in Cairo, including another in Tahrir Square that killed two foreign tourists and an Egyptian. More than 175 people have been killed since the beginning of last year in the terrorist attacks and gunbattles between police and radicals.

Since the assassination of Mr. Sadat, the highest-ranking official to be slain by fundamentalists was the speaker of the Parliament, Rifaat Mahgoub, in 1990. On Saturday, a civilian court here found 24 militants not guilty of the crime, denouncing the torture that had coerced their confessions.